Tisbury voters will weigh in at a special town meeting Tuesday on nearly $26 million in additional borrowing to complete the Tisbury School renovation project.

The meeting takes place Sept. 20 at 7 p.m. in the Martha’s Vineyard Performing Center in Oak Bluffs, which the town is renting for the occasion because preliminary demolition has already begun at the Tisbury School gym.

Tisbury voters approved borrowing $55 million for the school project in 2021, but since then construction costs have ballooned. The total project budget is now $81,843,284, according to an overview from Tisbury School owner’s project manager Daedalus/CHA, which represents the town.

At a joint public meeting on Tuesday of the Tisbury School committee and the school building committee, construction manager WT Rich Co. committed to absorbing any construction costs above the $70 million guaranteed maximum price (GMP). The additional $11,843,284 for the project includes architecture and design costs, contingency funds and various other expenditures.

As construction manager, WT Rich is solely responsible for controlling subcontractors’ costs. If the project comes in for less than $70 million, the company — not the town — keeps the change.

“We don’t anticipate it’s going to go any higher … That’s it,” WT Rich project manager Harvey Eskenas said Tuesday at a joint public meeting of the Tisbury School committee and the school building committee.

The company is bonded for performance and payment, Mr. Eskenas said.

“In case we default, the bond company ensures the job is completed on your behalf,” he told school officials.

The GMP agreement remains a draft until it has formally been approved by the town select board. The document has not been made public yet because some bidding is still under way, school committee chair Amy Houghton told the Gazette.

Tuesday’s joint meeting was also a hybrid gathering, with the school committee and some building committee members gathering at the town emergency services building on Spring street and others joining online. Speaking via Zoom, Mr. Eskenas gave a preview of next steps at the gymnasium, where work is currently on hold pending the town meeting vote.

“We will have to fence off the rest of the area to make sure it’s safe,” he said.

After that, the gym will be detached from the main school and taken down, which Mr. Eskenas estimated would take about a month.

Then the digging will begin.

“We’ll be … taking out several feet of soil so we can get down to the foundation grade of the new gym and the new cafeteria,” he said.

The entire process will be an orderly one, Mr. Eskenas said, in answer to a question from Tisbury School teacher and building committee member Rita Jeffers about what level of disruption the school community can expect.

“It’s not going to be a free-for-all with a bunch of machines going crazy,” he said. “It’s going to be very methodical.”

Children are still attending classes at the old school while temporary modular classrooms are made ready for an expected November move, which will take place regardless of the outcome of Tuesday’s vote.